fix: add typed Python SDK errors

[seans-openclawbot]

Summary

• add GlowBackError, DataError, and BacktestError to the supported Python SDK surface
• map Rust data and backtest backend failures to domain-specific Python exceptions while preserving the original error context
• add exception hierarchy/context regression tests and document caller-facing handling

Validation

• cargo test -p gb-python --locked --no-default-features
• cargo build -p gb-python --locked
• ./scripts/python_sdk_quickstart.sh
• python -m mkdocs build --strict

Refs #107